In general, among machine tools for processing a workpiece, a numerical control (NC) lathe includes a spindle unit of which shaft is operated in cooperation with a spindle motor and in which a chuck for clamping a workpiece is fit onto the shaft, a tail stock unit positioned on the central line identical to that of a drive shaft of the spindle unit to support the circumferential center of the workpiece so as to prevent the radial deviation of the workpiece from the reference position, i.e., the central axis of the workpiece due to the centrifugal force caused by the high rotation of the workpiece, a ball screw unit mounted on a bed positioned at the upper portion of the spindle unit and adapted to move horizontally and vertically to translate rotational motion to linear motion, and a tool post mounted over a carrier of the ball screw unit and including a variety of processing tools which can approach the workpiece to perform the machining operations such as, for example, turning, milling and drilling on the workpiece.
Also, the tool post is classified into a turret type which is mounted radially externally from the turret to machine the workpiece in a partitioning rotation manner according to a desired machining pattern and a gang type in which tools are arranged linearly to perform a desired machining on the workpiece through a vertical ascending and descending movement.
A turret device used in the turret-type tool post is configured such that various kinds of cutting tools are radially mounted to the circumference of a rotational cutting tool post called “a turret”. Whenever such a turret rotates by a predetermined angle, respective cutting tools sequentially perform a cutting machining on a to-be-cut workpiece. Accordingly, despite a machining part requiring multiple processes, if a plurality of cutting tools is sequentially mounted to the turret, the machining operation on the machining part can be accomplished by a one-time rotation of the turret.
Meanwhile, a machine tool equipped with the turret as constructed above is controlled by the following conventional control method.
FIG. 1 is a block diagram illustrating the inner construction of a turret servo control device according to the prior art.
As shown in FIG. 1, the turret servo control unit performs the overall operation of the machine tool equipped with the turret, and serves to control the operation of the turret-equipped machine tool according to a command preset by a numerical control program.
The conventional turret servo control device includes a control panel 10, an NC 20, a PLC 30, a servo driver 40 and a servo motor 50. When a control command is input to the turret servo control device through the control panel 10, the NC 20 transmits a command for rotating the turret to the PLC 30. Then, the PLC 30 generates an operating signal based on the contents programmed in the PLC 30 for application to the servo driver 40 which in turn controls electric power so as to effect a rotational operation of the servo motor 50.
Such a conventional turret servo control device for a machine tool allows the command or information from the PLC 30 to be input to the servo driver 40 to operate the turret without a speed change during the rotation of the turret.
However, the conventional turret servo control device entails a problem upon the checking of the interference between a base metal and a processing tool prior to the processing of the base metal after mounting the tool to the turret due to such a disadvantageous characteristic that the speed of the turret during its rotation cannot be altered. That is, when the rotation speed of the turret is as high as the exchange speed of the tool during the processing of the base metal, there is a risk that the tool mounted to the turret and the base metal may collide with each other at the time of checking the interference between the base metal and the tool. Thus, since it is impossible to check the interference between the base metal and the tool during the automatic rotation of the turret, an operator must make the turret held at an unclamped state and then directly turn it with his hand to check the interference between the base metal and the tool. Such manual operation causes danger and inconvenience to the operators in the process of treating tools.
In addition, a company which has purchased the NC must buys a set due to lack of compatibility of the PLC, which leads to an increase in the manufacturing cost.
That is, since the conventional turret servo control device is not equipped with a command processing means therein, the NC and the PLC including a command system associated with the servo driver must be purchased simultaneously.